Recently, a process for the low pressure chemical vapor deposition (LPCVD) of silicon nitride films from ammonia and bis-tertiary-butyl amino silane (BTBAS), having the formula (t-C4H9NH)2SiH2, has been proposed in U.S. Pat. No. 5,874,368 to Laxman. In addition to improved film properties, a significantly lower deposition temperature can be used as compared to a conventional process using ammonia and dichlorosilane (DCS), subjecting the semiconductor substrates to a significantly lower thermal budget. The process is operated in a conventional vertical furnace wherein a plurality of wafers, accommodated in a wafer boat in a horizontal position and in a vertically spaced relationship, are processed in a quartz process tube.
However, this BTBAS process has a large disadvantage. After a few deposition runs, or a limited cumulative film thickness of about 300 nm on the quartz process tube and the quartz wafer boat, the particle levels in the process tube start to increase to unacceptably high levels. In order to reduce the particle levels, the quartzware needs to be cleaned, and cleaned relatively frequently. Due to the high frequency of cleaning, this manufacturing process is only economical when the cleaning can be performed in-situ, by feeding a cleaning gas into the process tube, without a need to dismount the process tube for each cleaning. One frequently used cleaning gas is NF3. Unfortunately, after cleaning with NF3, the deposition rate of the BTBAS process appears to be significantly lower than before the cleaning, perhaps due to undesirable roughening of the exposed quartz surfaces. This effect can be counteracted by performing a pre-coating run using BTBAS without product wafers in the process tube. However, the pre-coating run contributes to the allowable cumulative deposition before a new in-situ cleaning becomes necessary. Thus, using BTBAS is counter-productive since it reduces the number of production runs that can be performed between cleanings.